App Pairs (behind flag): Add new ItemInfo types and DB save functionality

This is the second of several patches implementing the App Pairs feature behind a flag.

This patch includes:
- AppPairsController, a new controller that will handle creation, launching, and deletion of app pairs
- ITEM_TYPE_APP_PAIR is a new type of FolderInfo (FolderInfo now can be either a folder or an app pair, and should probably be renamed to CollectionInfo or something more generic in future)
- Necessary plumbing for these new types
- Database code that handles saving a new app pair to the database with the correct schema

Flag: ENABLE_APP_PAIRS (set to false)
Bug: 274189428
Test: Not included in this CL, but will follow
Change-Id: Ie3aefd4eb9171f471789f54876de742849d3013b